Whispering Gallery Modes: Harnessing Light on a Nanoscale

What are Whispering Gallery Modes?

Whispering gallery modes (WGMs) are a fascinating phenomenon that occurs in both optical and acoustic systems when waves are confined and circulate within a circular or spherical structure. The term "whispering gallery" originates from the acoustic phenomenon observed in the dome of St. Paul's Cathedral in London, where a whisper along the wall can be heard clearly on the opposite side due to the sound waves traveling along the curved surface.
In the optical domain, WGMs occur when light is trapped and circulates within a microresonator or a nanoresonator. These optical WGMs have gained significant attention in recent years due to their unique properties and potential applications in various fields of nanotechnology and photonics.
While the principles behind acoustic and optical WGMs are similar, the focus of this article will be on optical WGMs and their applications in nanoscale systems.
Illustration of whispering gallery modes in a spherical resonator
Map of a whispering gallery wave in an enclosed cylinder with of air of the same diameter as the whispering gallery at St Paul's Cathedral. Red and blue represent higher and lower air pressures, respectively, and the distortion in the grid lines represents the air particle displacements. The pitch of this wave is 69 Hz. (Image: Applied Solid State Physics Laboratory, Division of Applied Physics, Faculty of Engineering, Hokkaido University)

Key Characteristics of Whispering Gallery Modes

WGMs exhibit several unique characteristics that make them attractive for various applications in nanotechnology and photonics:
  • High Quality Factor (Q-factor): WGMs can have extremely high Q-factors, which is a measure of the energy stored in the resonator relative to the energy lost per cycle. High Q-factors enable long photon lifetimes and narrow linewidths, making WGMs suitable for sensitive sensing and high-resolution spectroscopy.
  • Small Mode Volume: WGMs are confined to a small mode volume near the surface of the resonator, resulting in a high intensity of the electromagnetic field. This strong field enhancement is beneficial for nonlinear optical effects, cavity quantum electrodynamics, and light-matter interactions.
  • Spectral Tunability: The resonance wavelength of WGMs can be tuned by changing the size, shape, or refractive index of the resonator. This tunability allows for the design of wavelength-selective devices and the adjustment of the resonance to match specific optical transitions or spectral regions of interest.

Resonator Geometries for Whispering Gallery Modes

Various resonator geometries can support WGMs, each with its own advantages and applications:

Microspheres and Nanospheres

Spherical resonators, such as microspheres and nanospheres, are the most common geometries for WGMs. These resonators can be fabricated from a wide range of materials, including silica, polymers, and semiconductors. The high symmetry of spherical resonators leads to degenerate modes and a dense mode spectrum, which can be advantageous for certain applications like sensing and frequency comb generation.

Microtoroids and Microdisks

Microtoroidal and microdisk resonators are planar geometries that offer a more integrated approach to WGM-based devices. These resonators are typically fabricated on a chip using lithographic techniques and can be easily coupled to waveguides or other photonic components. Microtoroids have a higher Q-factor compared to microdisks due to their smoother surface and reduced scattering losses.

Microbottles and Microbubbles

Microbottle and microbubble resonators are elongated geometries that support WGMs along their equatorial plane. These resonators offer unique advantages, such as a larger mode area and the ability to support multiple WGMs with different axial mode numbers. Microbottles and microbubbles are particularly useful for applications that require a large interaction length or the simultaneous excitation of multiple WGMs.

Excitation and Coupling of Whispering Gallery Modes

To excite WGMs in a resonator, light must be coupled into the structure efficiently. Several coupling techniques are commonly used:

Tapered Fiber Coupling

Tapered optical fibers are widely used for coupling light into WGM resonators. By bringing a tapered fiber close to the resonator surface, the evanescent field of the fiber can overlap with the WGM, allowing for efficient energy transfer. Tapered fiber coupling is a versatile and controllable method that enables the excitation of high-Q WGMs and the extraction of the circulating light.

Prism Coupling

Prism coupling relies on the evanescent wave coupling between a high-refractive-index prism and the WGM resonator. By adjusting the angle of incidence and the gap between the prism and the resonator, phase matching can be achieved, leading to efficient coupling. Prism coupling is particularly useful for resonators with a high refractive index or for applications that require a free-space optical input.

Integrated Waveguide Coupling

In integrated photonic devices, WGMs can be excited using on-chip waveguides. By designing the waveguide to match the phase and mode profile of the WGM, efficient coupling can be achieved. Integrated waveguide coupling enables the realization of compact and scalable WGM-based devices, such as optical filters, switches, and modulators.

Applications of Whispering Gallery Modes

WGMs have found numerous applications in various fields of nanotechnology and photonics:

Sensing and Biosensing

The high Q-factor and small mode volume of WGMs make them extremely sensitive to changes in the surrounding environment. By functionalizing the resonator surface with specific receptors or biomarkers, WGM-based sensors can detect minute changes in the refractive index or the presence of target analytes. WGM sensors have been used for label-free detection of proteins, viruses, and chemicals, as well as for monitoring physical parameters like temperature and pressure.

Nonlinear Optics and Frequency Comb Generation

The strong field enhancement in WGM resonators can significantly enhance nonlinear optical effects, such as second-harmonic generation, third-harmonic generation, and four-wave mixing. By exploiting these nonlinear effects, WGM resonators can be used for the generation of optical frequency combs, which have applications in metrology, spectroscopy, and optical communications.

Cavity Optomechanics

WGM resonators can also support mechanical vibrations, leading to the field of cavity optomechanics. The interaction between the optical and mechanical modes in a WGM resonator enables the study of fundamental quantum phenomena, such as the cooling of mechanical oscillators to their quantum ground state. Cavity optomechanics with WGMs has potential applications in quantum information processing, precision sensing, and the exploration of quantum-classical boundaries.

Challenges and Future Perspectives

Despite the remarkable progress in WGM research, several challenges need to be addressed for the widespread adoption of WGM-based devices. One of the main challenges is the reliable and scalable fabrication of high-quality WGM resonators with precise control over their size, shape, and material properties. The development of advanced fabrication techniques, such as 3D nanoprinting and self-assembly, may provide new opportunities for the large-scale production of WGM resonators.
Another challenge lies in the integration of WGM resonators with other photonic components and systems. The efficient coupling of light into and out of WGM resonators, as well as their compatibility with standard photonic platforms, is crucial for the realization of practical WGM-based devices. The development of novel coupling schemes and the integration of WGM resonators with waveguides, lasers, and detectors will be essential for future applications.
Future research directions in WGMs include the exploration of new materials and hybrid resonator systems that combine the advantages of different platforms. For example, the integration of WGM resonators with plasmonic nanostructures or two-dimensional materials like graphene may lead to enhanced light-matter interactions and novel functionalities. Additionally, the investigation of quantum effects in WGM systems, such as single-photon sources, entangled states, and quantum sensors, will open up new avenues for quantum technologies.
As the field of nanotechnology continues to advance, WGMs will play an increasingly important role in the development of ultrasensitive sensors, efficient nonlinear optical devices, and quantum photonic systems. The unique properties of WGMs, combined with the progress in nanofabrication and photonic integration, will undoubtedly lead to new breakthroughs and applications in the years to come.

Further Reading